The present invention relates to an improved apparatus for detecting the presence of an object, e.g. an intruder, within a given area. More particularly, the present invention relates to an improved apparatus for detecting the presence of an intruder within a given area by detecting the change in electrostatic charge caused by the disturbance by the intruder of the electric field within the given area.
Various types of intrusion detection systems are known in the art. One type of intrusion detection system which has been proposed is to utilize a high impedance sensing device, for example, an antenna, which is placed within the area to be monitored and produces an electric field therein, and then to detect any change in the charge on the antenna due to the electric field being disturbed by the intruder. The change in charge is then converted to an electrical signal which is used to provide an indication.
Although the prior proposed systems operate in theory, a number of difficulties arise when it is attempted to use these systems in practical applications. These difficulties arise as a result of the particular type of signal which is being detected, which is very small. Consequently, the charge detecting circuitry must be very sensitive so that it can respond to the very small signals produced by the induced charges or a very large antenna must be used. Additionally, when using such circuits in a convential environment, for example, either in or in the vicinity of a home or building, for the detection of an intruder, a problem arises as a result of random electric fields in the surrounding area which can likewise induce charges in the antenna. For example, one type of changing electric field which is commonly present in buildings is that resulting from the 60 Hz electrical power in the building. The fields from the power lines and other electrical apparatus connected thereto tend to saturate the detecting circuitry, thus preventing any effective detection of the relatively small charges induced by intruders, or at least tend to produce erroneous indications. As a result of these limitations, the intrusion detection systems previously proposed are severely limited in their use.